Question

I am trying to get contents of http://www.yahoo.com using WebClient#DownloadStringAsync(). However as Silverlight doesn’t allow cross domain calls i am getting TargetInvocationException. I know we have to put clientaccesspolicy.xml and crossdomain.xml in our web server root but that is possible only if i have control on my services. Currently Google is not under my control ;), so how do i handle it?

I’ve did a workaround by making a WCF service in my web application and then calling WebClient. This works perfectly but it is rather ineffecient. Is there any other better way than this?

Thanks in advance 🙂

Answer 1

Silverlight’s cross domain restricitions cause many developers to implement workarounds. If you need to display the html page you get back you should look into Silverlight 4 (WebBrowser) control although this only seems to work when running out-of-browser mode.

If you need to parse through the content you can try some of the following:

1. For a managed code solution the proxy service you have already implemented is your best option.

2. Write a Java applet that returns this information. Silverlight can interopt to javascript which can interopt into Java applets. This also works in the reverse but a little difficult to setup. (If you need more info on this let me know).

3. Use javascript XmlHttpRequest to get the data you want from the source. This can be difficult when supporting multiple browsers. This link shows an example of how to do this (you will need to scroll down). Javascript get Html

Code:

var xmlHttpRequestHandler = new Object();
var requestObject;

xmlHttpRequestHandler.createXmlHttpRequest = function(){
var XmlHttpRequestObject;
if(typeof XMLHttpRequest != "undefined")
{
    XmlHttpRequestObject = new XMLHttpRequest();
}
else if(window.ActiveXObject)
{
  var tryPossibleVersions =["MSXML2.XMLHttp.5.0", "MSXML2.XMLHttp.4.0", "MSXML2.XMLHttp.3.0", "MSXML2.XMLHttp","Microsoft.XMLHttp"];
    for(i=0;i<tryPossibleVersions.length;i++)
    {
        try
        {
            XmlHttpRequestObject = new ActiveXObject(tryPossibleVersions[i]);
            break;
        }
        catch(xmlHttpRequestObjectError) 
        {
            // Ignore Exception
        }
    }
}
return XmlHttpRequestObject;}

   function getHtml(){
  var url = document.getElementById('url').value;
  if(url.length > 0)
  {
        requestObject = xmlHttpRequestHandler.createXmlHttpRequest();
        requestObject.onreadystatechange=onReadyStateChangeResponse;
        requestObject.open("Get",url, true);
        requestObject.send(null);
  }}
function onReadyStateChangeResponse(){
  var ready, status;
  try
  {
        ready = requestObject.readyState;
        status = requestObject.status;
  }
  catch(e) {}
  if(ready == 4 && status == 200)
  {
        alert(requestObject.responseText);
  }}